This invention relates to a fuel supply control method for electronically controlling the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to a fuel supply control method of this kind, which is adapted to apply air/fuel ratio control coefficients for control of the air/fuel ratio in a manner such that the values of such coefficients are set to respective suitable values while the engine is operating in a plurality of particular operating regions, so as to control the air/fuel ratio to predetermined desired values or values close thereto in these particualr operating regions, thereby improving the operational stability of the engine as well as the driveability of same.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by Japanese Patent Provisional Publication (Kokai) No. 57-210137, which is adapted to determine the fuel injection period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed fuel control system, while the engine is operating in a normal operating condition, the air/fuel ratio is controlled in feedback mode such that the valve opening period of the fuel injection device is controlled by varying the value of a coefficient in response to the output from an exhaust gas ingredient concentration detecting means which is arranged in the exhaust system of the engine, so as to attain a theoretical air/fuel ratio or a value close thereto (closed loop control), whereas while the engine is operating in one of particular operating conditions (e.g. an idling region, a mixture-leaning region, a wide-open-throttle region, and a fuel-cut effecting region), the air/fuel ratio is controlled in open loop mode by the use of a mean value of values of the above coefficient applied during the preceding feedback control, together with an exclusive coefficient corresponding to the kind of operating region in which the engine is then operating, thereby preventing any deviation of the air fuel ratio from a desired air/fuel ratio due to variations in the performance of various engine operating condition sensors and a system for controlling or driving the fuel injection device, etc. and/or due to aging changes in the performance of the sensors and the system, and also achieving required air/fuel ratios best suited for the respective particular operating conditions, to thus reduce the fuel consumption as well as improve the driveability of the engine.
However, even with the above method of applying a mean value of values of the feedback control correction coefficient to air/fuel ratio control in such particular operating regions, the resulting air/fuel ratios can sometimes be largely deviated from respective desired air/fuel ratios during operation of the engine in these particular operating regions, because there are some differences in operating condition of the engine between the feedback control region and the particular operating regions, which makes it difficult to control the air/fuel ratio so as to optimize the emission characteristics, fuel consumption, etc. of the engine throughout all the particular operating regions. Further, when the engine is operating in such particular operating regions, particularly in the idling region, the engine can have its emission characteristics and fuel consumption rate largely affected even by a small change in the air/fuel ratio of the mixture supplied to the engine, thereby requiring strict and accurate control of the air/fuel ratio during operation of the engine in these particular operating regions, especially in the idling region.